Fuel cells, and particularly PEMFCs, are relatively attractive due to their high theoretical efficiency and to the non-polluting nature of the reaction byproducts.
Further, they provide a large power range, which makes it possible to envisage their use for stationary applications, such as high-power installations for electric power generation, as well as for electric vehicles, or any other device requiring an autonomous power source (electric generating unit, portable electronic device, etc.).
Generally, a proton exchange membrane fuel cell operates by oxidation of the fuel (hydrogen or methanol, for example) at the anode, and by proton transfer from the anode compartment to the cathode compartment through the proton exchange membrane. The electrons resulting from the oxidation reaction are conveyed back to the cathode via an external circuit.
Thus, the chemical energy may be converted into electric energy and into heat.
PEMFC fuel cells have many advantages, such as a lack of sensitivity to carbon dioxide; a low operating temperature which allows a fast start; a flexibility of use and of heat management; a decrease of electrode corrosion problems; and a lack of leakage of the electrolyte.
However, they also have disadvantages such as, for example, a high sensitivity to carbon monoxide; a low operating temperature (lower than 100° C.), which does not enable to use the heat; and an expensive catalyst (generally based on platinum) and membrane.
Preferably, the membrane (polymer electrolyte) of a PEMFC is impermeable to gases. It advantageously has good mechanical properties and a high proton conduction. Further, the forming of shapes adapted to the different applications should be possible, especially to be able to form thin systems (thickness of a few microns). Finally, the membrane is made of electrochemically and chemically stable polymer.
It may in particular be a polymer membrane based on ionomers of perfluorosulfonate type (PFSA), such as Dupont's Nafion® and Solvay Specialty Polymers' Aquivion®. In such perfluorosulfonate ionomers, the proton conductivity of the membrane is ensured by —SO3H groups (sulfonic acid function).
However, such membranes have disadvantages due to their permeability to methanol and to hydrogen. Further, their mechanical properties degrade beyond their optimal operating temperature (80° C.).
This is particularly constraining for the automobile field, for example. Indeed, for this type of application, a PEMFC operating between −30 and 120° C., and in the presence of slightly humidified gases (between 0 and 50% of relative humidity) is required.
The performance of a PEMFC is also linked to the following issues:
the presence of carbon monoxide (CO) generally causes a poisoning of the catalysts. When the hydrogen (fuel) is obtained by reforming, it generally contains traces of carbon monoxide. The presence of CO lowers the efficiency of the platinum-based catalyst which adsorbs it. The performances of the PEMFC are thus lowered. On the other hand, the adsorption of CO on the platinum-based catalyst is favored at low temperature, but affected at high temperature due to the negative entropy of the adsorption reaction. Thus, the tolerance to CO increases with temperature. The performance degradation of the PEMFC due to the CO poisoning may thus be significantly attenuated at high temperature (approximately 140° C.).
the thermal management of a PEMFC is more complicated at low temperature, given that the cell generates from 40 to 50% of its energy in the form of heat. Accordingly, when the cell operates at low temperature, large quantities of energy have to be dissipated. Conversely, when the cell operates at temperatures in the range from 120 to 140° C., the heat generated by the cell enables to maintain the system temperature and requires smaller cooling systems. This point is particularly important for an application in the automobile industry. Further, for temperatures higher than 100° C., the generated heat may also be used for other purposes (heating in cogeneration mode, for example).
the humidification of the membrane is essential at low temperature, given that PFSA-type membranes require being constantly hydrated. The additives necessary for the humidification complicate and decrease the reliability of the system. The humidification is necessary given that the proton conductivity of the membrane increases with the quantity of water contained in the polymer matrix, which itself increases with the quantity of water outside of the membrane (relative humidity). Such a humidification is all the more complex to achieve and to manage and requires all the more energy as the temperature is high.
As a summary, there is a need to develop a PEMFC membrane capable of being used at low temperature as well as at high temperature with gases having a low water content (<50% of relative humidity).
To overcome these problems, the Applicant has designed a proton exchange membrane having a better thermomechanical stability at high temperature than prior art membranes, and this, without altering the proton conductivity of said membrane.
The technical problem solved by the present invention thus is to improve the thermomechanical properties of a fuel cell membrane, thus capable of being used in a larger temperature range than prior art membranes, thus allowing a use with a low relative humidity.